fix(lsp): :lsp restart restarts on client exit #37125

Problem:
`:lsp restart` detects when a client has exited by using the `LspDetach`
autocommand. This works correctly in common cases, but breaks when
restarting a client which is not attached to any buffer. It also breaks
if a client is detached in between `:lsp restart` and the actual
stopping of the client.

Solution:
Move restart logic into `vim/lsp/client.lua`, so it can hook in to
`_on_exit()`. The public `on_exit` callback cannot be used for this, as
`:lsp restart` needs to ensure the restart only happens once, even if
the command is run multiple times on the same client.

runtime/lua/vim/lsp/client.lua

@@ -908,6 +908,31 @@ function Client:stop(force)
   end)
 end
 
+--- Stops a client, then starts a new client with the same config and attached
+--- buffers.
+---
+--- @param force? integer|boolean See [Client:stop()] for details.
+---                               (default: `self.exit_timeout`)
+function Client:_restart(force)
+  validate('force', force, { 'number', 'boolean' }, true)
+
+  self._handle_restart = function()
+    --- @type integer[]
+    local attached_buffers = vim.tbl_keys(self.attached_buffers)
+
+    vim.schedule(function()
+      local new_client_id = lsp.start(self.config, { attach = false })
+      if new_client_id then
+        for _, buffer in ipairs(attached_buffers) do
+          lsp.buf_attach_client(buffer, new_client_id)
+        end
+      end
+    end)
+  end
+
+  self:stop(force)
+end
+
 --- Get options for a method that is registered dynamically.
 --- @param method vim.lsp.protocol.Method | vim.lsp.protocol.Method.Registration
 function Client:_supports_registration(method)
@@ -1334,6 +1359,11 @@ function Client:_on_exit(code, signal)
     end
   end)
 
+  if self._handle_restart ~= nil then
+    self._handle_restart()
+    self._handle_restart = nil
+  end
+
   -- Schedule the deletion of the client object so that it exists in the execution of LspDetach
   -- autocommands
   vim.schedule(function()